fix: grouped lists show their cards on iOS

[SRWieZ]

A native:list with list-sections is supposed to look like the iOS Settings app: each section is a white rounded card on a grey background. Android draws those cards. On iOS they never show: the renderer clears every row's background, so the rows float on the grey background with no card.

This PR keeps the system card colour on grouped rows (light and dark mode follow the system). Plain lists are unchanged; their rows stay clear.

It also adds an opt-in row-color on the list, for apps with a coloured background that want their own card colour (white cards on a beige screen, for example). It accepts a hex value or a theme token. It works on both platforms and in both styles.

To reproduce:

<native:column class="w-full h-full">
    <native:top-bar title="Settings" display-mode="inline" />
    <native:list class="w-full flex-1" separator>
        <native:list-section header="General" footer="Rows in a section share one rounded card.">
            <native:list-item headline="Notifications" :trailingSwitch="true" />
            <native:list-item headline="Sounds" :trailingSwitch="false" />
            <native:list-item headline="Language" trailingText="English" />
        </native:list-section>
        <native:list-section header="Account">
            <native:list-item headline="Profile" />
            <native:list-item headline="Sign out" />
        </native:list-section>
    </native:list>
</native:column>

Before / after (iPhone 17 Pro simulator, iOS 26.5):

With a coloured screen and row-color:

<native:column class="w-full h-full bg-[#F8E9D3]">
    <native:list class="w-full flex-1" transparent row-color="#FFFFFF" separator>
        …same sections…
    </native:list>
</native:column>